The invention concerns a device according to the precharacterizing clause of Claim 1.
Known from the prior art are manifold devices that display at least one sensor arranged inside the body of the patient for receiving a body-specific measurement signal containing information concerning demand for heart performance, in order to derive therefrom, among other things, a signal for influencing the stimulation rate.
Devices in existence until now started out from the fact that there exists a direct and time-correct relationship between the change in the body-specific measurement signal containing information concerning demand for heart performance and the stimulation magnitude to be influenced, as for example the amplitude, the rate or the average frequency or occurrence of signals picked up by a sensor, and the heart rate.
However, it has been shown that a direct processing of this type in the time or frequency domain often does not lead to the desired results.
Therefore, the task underlying the invention is to improve the controlling of stimulation events and, more particularly, of the heart rate.
The task, starting out from a device based on the precharacterizing clause in Claim 1, is accomplished by the features given in the characterizing part of Claim 1.
The invention includes the technical teaching that in the determination of a magnitude influencing the stimulation signal sequence, those relationships also should be observed that extend beyond a direct association. Relating to this, on the one hand, is a transformation of the time periods to be considered in such a manner that periodic observation also takes place over past time periods, and that control of the heart rate is checked by the spatial-temporal displacement of complex frequency sequences or event sequences that show time delays within the body. These time delays are obtained through the temporal displacement of two measurement-values that are picked up in a spatially displaced manner. The agreement of the underlying event signals is preferably verified here by pattern comparison. The time delays of the frequency or event sequences are determined here by the circulatory dynamic and the body-internal regulating processes underlying this.
In this manner, it is possible to xe2x80x9cdecodexe2x80x9d a body-specific dynamic and to make it accessible for heart stimulation, which dynamic, to be sure, until now has already played an important role in controlling the vasomotor system of the human body, but could not be used for reconstructing natural heart rhythm.
In the case of the signals to be evaluated, we are dealing with circulation magnitudes whose constant fluctuation in dynamic variations also contains information concerning demand for heart performance. Here, we are dealing specifically with time delays of electrical signals in the heart. In particular, characteristic here is the Axe2x80x94A conduction, which, when it exceeds or falls below characteristic values, is an early sign of a threatening tachycardia or fibrillation. That the time-delay difference between the two atriums is too large must be recognized early enough to prevent through biatrial stimulation the initiation of a tachycardia. Thus, by evaluation of the corresponding signal time-delay, countermeasures that prevent a corresponding state, for example biatrial stimulation sequences, can be taken early enough.
Associated with the invention is the important advantage that, for the first time, signals that until now were not capable of being detected, or that remained unobserved, co-determine the stimulation rate, so that sensors detecting the physical activity of the patient can be dispensed with.
The use of the device in accordance with the invention for maintaining or restoring a natural heart rhythm by generating an electrical stimulation signal relates, in particular, to implantable heart pacemakers for treating bradycardia or tachycardia, as well as corresponding hearty-rhythm correcting devices, which are also in use as implantable defibrillators.
If, according to the invention, there is connected to the downstream side of a sensor a processing unit that obtains from the measurement signal a body-specific time-delay signal whose duration covers a range from a few milliseconds up to a second, whereby the body-specific time-delay signal forms at least indirectly a control signal that influences the point in time, or the time sequence and/or the point in time, of the stimulation signal, then this means that information is taken from a time-delay signal impressed on the circulation system or the nervous system that is of significance for heart activity. It has been found that these signals are also of significance for human heart activity, and a stimulation in correlation to such signals forms at least a physiological supplement to signals that are relevant for heart function and picked up some other way from the patient""s body.
The selection of the body-specific time-delay signal can be accomplished by means of digital filtering or use of a correlation technique.
In the case of a digital processing, there also exists the possibility of effecting, in advantageous fashion, a detection or a synchronization of the time-delay processes by comparison of complete signal patterns within the amplitude and/or frequency range. Here, in particular in the processing unit, selected by means of a time window within a time segment of a predetermined duration comprising less than 100 milliseconds, is at least one periodically-occurring signal, or a corresponding portion of a signal, having a characteristic amplitude response or frequency pattern, and derived from the sequence of the appearance of this pattern is the body-specific time-delay signal that influences the time sequences or the point in time of the stimulation signal. The width of the time segments to be used in this processing corresponds here to the time delays of the expected signals or signal portions. A determination of a control signal based on the similarity or matching of amplitude or frequency patterns appearing in the time delay includes the advantage that a similarity of the signal portions determining the time-delay frequency is recognized more rapidly and more reliably even in the case of less-frequently repeating periods, and the xe2x80x9cbuild upxe2x80x9d of filter circuits or the like need not be waited upon in order to detect the periodic signal portions. This is particularly convenient in the case of longer delay times.
Here, for detection of coincident signal patterns, is stored in particular the periodically appearing signal or a corresponding signal portion picked up in each case in the time segment, having a characteristic amplitude response or frequency pattern, whereby the pattern of the current time segment is compared with at least one similar periodically-occurring signal picked up earlier or with a corresponding signal portion having a characteristic amplitude response or frequency pattern and, in coincidence with the later-appearing, periodically-occurring signal or corresponding signal portion having a characteristic amplitude response or frequency pattern, a control signal is emitted when the degree of conformity of the patterns to be compared with one another exceeds a predetermined value, whereby derived from the signal indicating the conformity is the control magnitude that influences the magnitude of the stimulation. In place of the frequency pattern, it is also possible here to compare the time response of one or several spectral portions in the frequency pattern that was (were) picked up within a time segment (wavelet).
The control magnitude, which influences the time sequence or the point in time of the stimulation signal, can here also be derived from the rate, the average frequency, the temporal change of the rate or of the average frequency, of the signal indicating conformity, so that there results a statistical compression of the derived information that is drawn upon for the stimulation. A corresponding compression of the information can also be achieved if the information from the amplitude response pattern and from the spectral pattern is evaluated together.
For generating frequency spectra, the time-dependent signal is appropriately subjected to a Fast-Fourier-Transformation (FFT).
Coming into consideration as body-specific sensor signals that are subjected to the processing in accordance with the invention are, in particular, characteristic portions of the intracardial electrogram, the Axe2x80x94A interval, the interarterial stimulus speed, and other heart-internal time delays.
Other advantageous developments of the invention are characterized in the dependent claims or are represented in more detail in the following with the aid of the figures, together with the description of the preferred implementation of the invention.